Apple Inc.‘s iOS, the mobile operating system that powers the iPad and iPhone, has more than doubled its share of the worldwide operating system market over the past 10 months.

When you count desktops, laptops, mobile phones and tablets as one category, iOS is the third most widely used operating system after Windows and Mac OS X, with share that jumped from less than nine-tenths of a percentage point in June 2010 to 2.24 per cent in April 2011, according to Net Applications.

The data, collected by determining the operating system of devices connected to the Web, shows Windows declining from 91.46 per cent to 88.91 per cent share since June. Mac OS X, Apple’s desktop operating system, gained a quarter of a percentage point in that time span but it’s iOS that is Apple’s real growth engine, moving from 1.87 per cent to 2.24 per cent in the past month alone. More than half of that comes from the iPhone, with the iPad and iPod Touch contributing the rest.

The total numbers by percentage are small, to be sure, but given Microsoft‘s long dominance of the OS market and sales of 350 million copies of Windows 7, only a fast-selling product like the iPad could make any kind of dent in total operating system market share. The iPad thoroughly dominates the newly emerging tablet category.

Even Android, which is more widely used than the iPhone, holds only 0.66 per cent of the OS market, putting Google’s mobile OS in sixth place behind Linux desktops and Java ME for mobile devices. Symbian, BlackBerry and Windows mobile are in seventh, eighth and ninth place, respectively, behind Android.

Microsoft’s Windows looks better when you exclude mobile devices from the picture. A recent Gartner study examining operating system revenue for desktops showed Windows capturing 96.6 per cent of the money, and 78.6 per cent when desktops and servers are lumped into one category.

But given the fact that mobile phones and tablets are replacing some of the tasks performed by desktops and laptops, there is growing momentum behind the idea of counting mobile operating systems in the same category as desktop software.

A recent study by Canalys, for example, said worldwide PC sales are up 7 per cent on the strength of iPad sales, and put Apple as the world’s fourth-largest PC manufacturer behind HP, Acer and Dell. In Q1 2011, Apple sold 8.5 million computers including Macs and iPads, according to Canalys.

Digging into the iOS numbers, Apple’s 2.24 per cent market share consists of 1.23 per cent for the iPhone, 0.82 per cent for the iPad and 0.19 per cent for the iPod Touch, according to Net Applications. iPhone share actually grew more than iPad share in the most recent month.

iOS grew faster than Mac OS between March and April when counting all of Apple’s mobile devices. But Macs actually grew faster than the iPad alone, moving from 5.25 per cent to 5.40 per cent, compared to the iPad’s 0.70 per cent in March and 0.82 per cent in April.

That is possibly indicative of Apple’s mobile success convincing some users to consider Macs instead of Windows computers. However, Mac has grown much more slowly than iOS when you look at it on a longer timeline. Since June 2010, Mac OS is up from 5.16 to 5.40 per cent of total OS usage. Apple still has a long way to go in the desktop market, but its overall success has helped the company push ahead of Microsoft in profit for the first time in two decades.